1. Field of the Invention
The present invention relates to an LED driving apparatus and an LED lighting apparatus employing the LED driving apparatus.
2. Description of Related Art
LED (light emission diode) elements are used for backlights of liquid crystal displays and light sources of lighting devices such as streetlights. Incandescent bulbs and fluorescent lamps are being replaced with bulb-type LED lighting apparatuses and fluorescent-lamp-type LED lighting apparatuses using white LED elements. These LED lighting apparatuses need LED driving apparatuses that are capable of correctly supplying electric power to the LED elements.
FIG. 1 is a circuit diagram illustrating an LED lighting apparatus 110 having an LED driving apparatus 101 according to a related art. The LED driving apparatus 101 includes a rectifier DB100, a transformer TR100, a switching element MOSFET Q101, a rectifying-smoothing circuit including a diode D101 and a capacitor C101, a controller CNT 100, and a capacitor C103. The LED driving apparatus 101 and an LED load 102 including LED elements LED1 to LEDn are the LED lighting apparatus 110.
The rectifier DB100 is a known diode bridge and is connected to an AC input power source AC100. The transformer TR100 includes a primary winding W101, a secondary winding W102, and a tertiary winding W103. A first end of the primary winding W101 is connected to the rectifier DB100 and a second end thereof is connected to a drain of the MOSFET Q101. Both ends of the secondary winding W102 are connected to the rectifying-smoothing circuit. Both ends of the tertiary winding W103 are connected to an auxiliary power source that includes a diode D102 and a capacitor C102.
A source of the MOSFET Q101 is grounded and a gate thereof is connected to the controller CNT 100. Both ends of the rectifying-smoothing circuit are connected to a series circuit that includes the LED load 102 and a detective resistor Rs100. The LED load 102 includes the n pieces of LED elements LED1 to LEDn that are connected in series. Here, the number of “n” is a natural number equal to or greater than 1.
The controller CNT100 has terminals that are connected to the gate of the MOSFET Q101, the auxiliary power source, a connection point of the LED load 102 and detective resistor Rs100, and the capacitor C103, respectively. The controller CNT 100 also has a terminal that is connected through a resistor R101 to a connection point of the rectifier DB100 and the primary winding W101 of the transformer TR100. The controller CNT100 includes an error amplifier AMP101 and a comparator CMP100.
An inverting input terminal (−) of the error amplifier AMP101 is connected to the connection point of the LED load 102 and detective resistor Rs100 and a non-inverting input terminal (+) thereof is connected to a reference voltage that includes a constant voltage source REG and a zener diode ZD101. An output terminal of the error amplifier AMP101 is connected through an integrator that includes a resistor R103 and the capacitor C103 to a first non-inverting input terminal of the comparator CMP100. The error amplifier AMP101 provides the comparator CMP100 with an error signal based on a current passing through the LED load 102 and a reference value.
An inverting input terminal of the comparator CMP100 is connected to a triangular wave generator and a second non-inverting input terminal thereof is connected to a duty limiter that includes the constant voltage source REG and resistors R104 and R105. An output terminal of the comparator CMP100 is connected through a driver to the gate of the MOSFET Q101. The comparator CMP100 provides the driver with a PWM signal based on the error signal from the error amplifier AMP101 and a triangular wave from the triangular wave generator.
In the LED driving apparatus 101, the controller CNT100 controls ON/OFF of the MOSFET Q101 in such a way as to equalize a current passing through the LED load 102 with a reference value (reference voltage), thereby adjusting a voltage applied to the LED load 102.
The related LED driving apparatus is disclosed in Japanese Unexamined Patent Application Publication No. 2004-527138 (Patent Document 1).